1. Field of the Invention
This invention relates to corrosion-resistant glassceramic-to-metal seals. In particular, the invention relates to plug seals for use in batteries. Plug seals are comprised of metal pin electrodes mounted by glassceramic-to-metal seals in surrounding metal collars.
The invention provides glass compositions for use in making corrosion-resistant, hermetic glassceramic-to-metal seals, as well as a method and apparatus for making plug seals.
The glass compositions for use in making corrosion-resistant, hermetic glassceramic-to-metal seals and having the property of sequentially fluidizing, sealing and ceraming during a single rapid heating step comprise about 25.0-32.0% by weight Zno, about 2.5-10% by weight Al.sub.2 O.sub.3 and about 30.0-60.0% by weight SiO.sub.2 together with optimal fluxing and/or nucleating agents.
The method of making a hermetic plug seal having a metal pin electrode mounted by a corrosion-resistant glassceramic-to-metal seal in a surrounding metal collar comprises:
(a) positioning a glass composition preform between and in proximity to the pin and the collar, within a controlled atmosphere enclosure;
(b) rapidly heating the metal pin and collar to cause the glass preform to heat, by conduction and radiation, so that it flows between and adheres to the pin and collar, and then cerams; and
(c) cooling the plug seal within the controlled atmosphere enclosure to room temperature.
The apparatus for making a hermetic plug seal having a metal pin electrode mounted by a corrosion-resistant glassceramic-to-metal seal in a surrounding metal collar comprises:
(a) a R.F. induction coil mounted about an oven body having gas inlet means and a loading port;
(b) a jig positionable within said oven body for holding a glass composition preform between and in proximity to the pin and the collar;
(c) a gas-permeable closure for sealing said loading port;
(d) a generator and source of power for said R.F. induction coil; and
(e) a source of inert gas in communication with said gas inlet means.
2. Description of the Prior Art
Plug seals, such as are used in Ni-Cd batteries, are well known in the art. See for example Fairweather et al Canadian Pat. No. 1,000,353 issued Nov. 23, 1976.
Most prior art plug seals however, when used in a corrosive environment such as the 30% KOH aqueous electrolyte used in Ni-Cd batteries, failed prematurely and/or released contaminants into the cell.
Prior art methods of making plug seals having glassceramic-to-metal seals involved two basic steps. The first step was the formation of a bond between the metal and the glass. This was accomplished by heating the glass to a temperature sufficiently high to cause the glass to flow and form a fluid seal. The second step was the ceraming of the glass to form a glassceramic-to-metal seal. The ceraming typically involved two heating steps in order to convert the glass into a glassceramic by catalytic crystallization. In the first heating step, crystallization nuclei precipitated at temperatures slightly above the softening point of the glass. In the second step, the temperature was raised in order to decrease the viscosity of the glass so that crystal growth occurred and the glass transformed into a glassceramic.
In view of short plug seal life in corrosive environments, and the relative complexity and hence cost of manufacturing plug seals, a need has existed for improved plug seals which are simple in design and easy and inexpensive to manufacture.
It has been found that certain glass compositions based on the ZnO-Al.sub.2 O.sub.3 -SiO.sub.2 system can be sequentially fluidized and ceramed in a single heating step to yield hermetic plug seals which exhibit good corrosion resistance in a KOH environment.
McMillan in "Glass Ceramics" (Academic Press, 1964) described glassceramics based on the Zno-Al.sub.2 O.sub.3 -SiO.sub.2 system as exhibiting high thermal expansion characteristics and electrical resistances. Subsequently, McMillan et al in "Glass Technology 7," pages 121 to 126, (1966) utilized glass compositions based on the ZnO-Al.sub.2 O.sub.3 -SiO.sub.2 system in making glassceramic-to-metal seals.
It was not however previously recognized that certain selected compositions from the ZnO-Al.sub.2 O.sub.3 -SiO.sub.2 system could be sequentially fluidized, sealed and ceramed in a single rapid heating step to yield plug seals exhibiting good corrosion resistance in a KOH environment.